1. Field of the Invention
The present invention generally relates to high-frequency modules, and more particularly to a high-frequency module suitable for a smaller, thinner, and lighter-weight electronic device.
2. Description of the Related Art
Smaller, thinner, and lighter-weight electronic devices such as portable mobile terminals have been developed at an increasing speed. The portable mobile terminals include high-frequency modules or high-frequency substrates represented by power amplifiers.
Therefore, it is also necessary to develop smaller, thinner, and lighter-weight high-frequency modules in order to obtain smaller, thinner, and lighter-weight portable mobile terminals.
FIGS. 1 and 2 are a schematic plan view and a schematic cross-sectional view of a conventional high-frequency module 1, respectively.
The high-frequency module 1 includes a high-frequency substrate 2, a high-frequency active chip 3 and a plurality of electronic components 4. The high-frequency substrate 2 includes a base material 15 made of ceramics, glass-ceramics, a glass fabric based epoxy resin or the like. The base material 15 has high-frequency circuit wiring lines 6 and 7, direct-current (DC) circuit wiring lines 8 and 9, and pad portions 12 through 14 each formed in a predetermined pattern on its upper surface, and a ground film 18 and land portions 19 formed on its lower surface. Hereinafter, the high-frequency circuit wiring lines 6 and 7 and the DC circuit wiring lines 8 and 9 are referred to as wiring lines 6, 7, 8 and 9, respectively.
An opening portion 16 is formed in a predetermined position in the base material 15 of the high-frequency substrate 2 having the above structure. Mounted in the opening portion 16 is the high-frequency active chip 3, which is electrically bonded to the wiring lines 6 through 9 by bonding wires 17.
The high-frequency substrate 2 has the electronic components 4 mounted thereon, which are joined to the respective wiring lines 6 through 9 by a conductive material such as a solder, a gold paste, or a silver paste. The pad portions 12 through 14 are electrically connected to the ground film 18 formed on the lower surface of the base material 15 by via holes (not shown) formed to penetrate the base material 15.
On the other hand, a high-frequency input terminal 22, a high-frequency output terminal 23, and bias terminals 10 and 11 (hereinafter referred to as terminals 22, 23, 10 and 11, respectively) are formed on the predetermined end portions of the wiring lines 6 through 9, respectively. The terminals 22, 23, 10, and 11 are electrically connected to the respective land portions 19 serving as terminals for external connection by via holes 20 formed to penetrate the base material 15. The land portions 19 are electrically connected to a mounting board when the high-frequency module 1 is mounted thereon. The upper surface of the high-frequency substrate 2 is sealed by a metal cap (not shown) or the like.
For example, Japanese Laid-Open Patent Application No. 11-017063 discloses a package for accommodating a semiconductor chip on which a semiconductor chip compatible with high frequencies, which is different from a high-frequency module, alone is mounted.
Conventionally, however, when ceramics, which is more expensive than a resin material, is employed as the base material 15 of the high-frequency substrate 2 provided for the high-frequency module 1, there arises a problem that the cost of the high-frequency module 1 rises.
Another problem with the conventional high-frequency module 1 is that when ceramics, glass-ceramics, a glass fabric based epoxy resin or the like is employed as the base material 15 of the high-frequency substrate 2, difficulty in reducing the thickness of the base material 15 to 100 xcexcm or less prevents the thickness of the high-frequency module 1 from being reduced.
Further, when ceramics or glass-ceramics is employed as the base material 15, it is difficult to freely obtain the diameter of each of the via holes 20 in desired size considering the burning shrinkage of a green sheet. When a glass fabric based epoxy resin is employed as the base material 15, it is also difficult to freely form the diameter of each of the via holes 20 in desired size, for there is a limit to the downsizing of the diameter of each of the via holes 20 because through holes to be formed into the via holes 20 are formed by machining.
As an electric circuit, each of the via holes 20 forms an equivalent circuit shown in FIG. 3 including an inductance L, an electrostatic capacity C and a resistance R because the high-frequency module 1 processes a high-frequency signal.
In the high-frequency module 1 processing the high-frequency signal, the equivalent circuit causes the high-frequency characteristic of the high-frequency module 1 to be deteriorated. The impedance of the equivalent circuit increases as the diameter of each of the via holes 20 becomes smaller or the base material 15 becomes thicker.
It is desirable to reduce the impedance of the equivalent circuit as much as possible. According to the conventional high-frequency module 1, however, such difficulties in freely obtaining the desired thickness of the base material 15 and the diameter of each of the via holes 20 in desired size prevent the impedance from being reduced, thus entailing the deterioration of the high-frequency characteristic of the high-frequency module 1 resulting from the via holes 20.
Such measures as to design the circuit of the high-frequency module 1 in consideration of the characteristic of the via holes 20 can be taken in order to preclude the deterioration of the high-frequency characteristics of the high-frequency module 1. However, it is difficult to securely remove the loss resulting from the via holes 20 because the characteristic of the equivalent circuit, as described above, varies depending on the diameter of each of the via holes 20 and the thickness of the base material 15. As the frequency of a signal to be processed in the high-frequency module 1 becomes higher, the high-frequency characteristic thereof becomes more deteriorated, thus causing such a circuit design to be extremely difficult especially in case of a high frequency.
Further, it is desired of the high-frequency module 1 to expand a bandwidth without changing the width of a signal line, which requires the base material 15 to be thinner and a relative dielectric constant to be lower.
However, when a conventional material such as ceramics, glass-ceramics or a glass fabric based epoxy resin is employed as the base material 15, the base material 15 becomes so thick and the dielectric constant becomes so high that it is difficult to expand the bandwidth without changing the width of the signal line. Therefore, when a circuit is designed in a millimeter wave region by employing ceramics, which has a high relative dielectric constant, as a base material, the width of a 50-xcexa9 signal line becomes so narrow as to be difficult to form.
Further, as a measure to radiate heat from a part to be mounted, or the high-frequency active chip 3, thermal vias 21 can be formed in the base material 15 as shown in FIG. 2. When the amount of heat radiated from the high-frequency active chip 3 is so large as to require the thermal resistance of the high-frequency substrate 2 to be reduced, it is possible to increase the number of the thermal vias 21 in order to radiate such amount of heat.
However, it is not desirable to increase the number of the thermal vias 21 in light of the strength of the high-frequency substrate 2. In other words, a problem lies in that the strength of the high-frequency substrate 2 decreases as the heat radiation characteristic is enhanced, while the heat radiation characteristic is deteriorated as the strength thereof increases.
It is a general object of the present invention to provide a high-frequency module in which the above disadvantages are eliminated.
A more specific object of the present invention is to provide a high-frequency module realizing excellent high-frequency and thermal characteristics and low production cost.
The above objects of the present invention are achieved by a high-frequency module including a substrate including a thin film resin sheet, a high-frequency circuit wiring line forming a first wiring layer, the high-frequency circuit wiring line formed on an upper surface of the substrate, a highfrequency circuit component provided on the upper surface of the substrate, and a resin sealing package formed on the upper surface of the substrate, the resin sealing package sealing the first wiring layer and the high-frequency circuit component.
By employing the thin film resin sheet as the substrate, the high-frequency module can be produced at lower cost compared with a conventional high-frequency module employing a ceramic as a base material for a substrate, and the excellent high-frequency characteristic of the high-frequency module can be realized in the broad bandwidth of a frequency.
The above objects of the present invention are also achieved by the high-frequency module further including the substrate having an opening portion formed in a position where the high-frequency circuit component is to be mounted, and a ground film formed on the lower surface of the substrate closing an opening of the opening portion, wherein the high-frequency circuit component is mounted on the ground film inside the opening portion.
According to the high-frequency module of the above structure, heat generated in the high-frequency circuit component is radiated to outer air through the ground film, thus enhancing the thermal characteristic of the high-frequency module.